The present invention relates to global positioning system (GPS) technology. In particular, the present invention relates to an improved technique for estimating the ionospheric delay in GPS signals.
The use of GPS technology has become increasingly prevalent in the world over the past several years. Basic GPS technology is well-known in the art and need not be described in depth herein, but for the purpose of background, its basic principles will be set forth.
GPS utilizes a constellation of satellites orbiting the Earth (28 total in operation with an additional 4 back-up satellites). The orbits of these satellites are arranged so that at any time, anywhere on Earth, 6 to 12 GPS satellites are visible by a GPS receiver. Each satellite broadcasts navigation signals of two frequencies, L1 at 1575.42 MHz and L2 at 1227.6 MHz. The signals are spread spectrum signals transmitted using code division multiple access (CDMA) techniques wherein each GPS satellite is assigned its own code. A GPS receiver operates to receive the signals transmitted by the GPS satellites. The coding present in the GPS satellite signals allows the GPS receiver to separate the different signals that correspond to each visible satellite. The coding also allows the GPS receiver to measure the signal propagation delay from the individual satellites and the GPS receiver. The signal delay along with a clock error shared by measurements to all satellites forms the GPS receiver""s estimate of pseudorange. All of these processes are well-known in the art.
The term xe2x80x9cpseudorangexe2x80x9d is used because this position estimate is obtained after making certain approximations regarding how the signal propagates from the satellite to the receiver. In actuality, there are several sources of error in this approximation, including ephemeris errors, clock errors, receiver noise, and most importantly for the present invention, atmospheric errors, all of which are well-known in the art. The atmospheric errors are due to the effects of both the troposphere and the ionosphere. The contribution of the troposphere to pseudorange error can be well accounted for using existing error modeling techniques. However, there is a need in the art for an improved method of correcting for ionospheric signal delay in pseudorange estimates.
The ionosphere ranges between 100 kilometers to 1000 kilometers above the Earth in altitude. The ionosphere is a dispersive medium that introduces errors greater during the day than at night and greater for satellites near the horizon than for satellites near zenith. Uncompensated iono-errors can range anywhere from 1 meter to more than 60 meters. Typical iono-errors are 10 meters for satellites near zenith and 30 meters for satellites near the horizon. In most GPS applications, the iono-error represents the most dominant source of error in the GPS position estimation.
A current approach used in commercial GPS receivers is to model the ionospheric delay in software using parameters transmitted every 12 minutes from the GPS satellites. This approach cuts the iono-error approximately in half. However, even at this lower error level, the ionosphere serves as a significant error source.
Differential GPS receivers and associated software have the capability of accurately estimating ionospheric delays. However, such delay estimation requires the availability of base stations that transmit reference signals. Such base stations currently do not have global coverage and are costly to implement. As such, the extension of this type of technology to non-differential GPS technology suffers from shortcomings.
Military GPS receivers (or dual-frequency GPS receivers) use measurements made simultaneously on both the L1 and L2 frequencies to correct iono-error (commercial GPS receivers can only operate on a single frequency). A typical error with this approach is 1-2 meters. At this level, the iono-error is commensurate with other GPS error sources. Including all error sources, a well-tuned military GPS receiver and associated software is typically capable of a total circular error probable (CEP) of approximately 4 meters. An example of a dual frequency approach to ionospheric delay estimation can be found in U.S. Pat. No. 5,876,411 issued Feb. 2, 1999 to Kumar, the entire disclosure of which is incorporated herein by reference.
However, it must be noted that the dual frequency approach is problematic in GPS jamming scenarios. Particularly in military applications, a GPS jamming device may be used to jam the L1 and L2 signals. GPS anti-jam (AJ) systems are available, but the cost of an AJ system capable of counter jamming on both the L1 and L2 frequencies is almost twice the cost of a system that can counter jam only one frequency at a time. As such, because it is much more cost-effective to leave a single GPS frequency unjammed at a given time than to unjam both frequencies, a strong need in the art exists for a GPS system that can substantially reduce iono-errors using only a single GPS frequency.
With this problem in mind, the inventors herein developed the present invention wherein the iono-error modeling software does not require simultaneous measurements in both the L1 and L2 frequencies. Because only a single GPS frequency is needed at any given time to produce highly accurate ionospheric delay estimates, the present invention is suitable for use in a wide variety of applications where jamming of GPS signals may occur, particularly GPS-guided munitions.
One aspect of the invention is to model the ionospheric delay between the GPS receiver antenna and each GPS satellite such that the ionospheric delay consists of a xe2x80x9cglobalxe2x80x9d delay (common to all GPS satellites) and xe2x80x9clocalxe2x80x9d delays (each applicable to an individual GPS satellite). The inventors herein have found that the accuracy of such an approach to ionospheric delay estimation is more accurate than the conventional dual frequency approach used in conventional military GPS technology and more accurate than single frequency approaches known to the inventors. The global iono delay is modeled as the delay that would be observed between the GPS receiver antenna and a GPS satellite for a satellite at a known position wherein the ionosphere is an ideal homogeneous layer at a predetermined height above the earth. The preferable known position of such a satellite is at zenith and the preferable predetermined height is 190 nautical miles.
Accordingly, disclosed herein is a method for estimating ionospheric delays present in pseudorange measurements produced by a global positioning system (GPS) receiver configured to track a plurality of GPS satellites, the method comprising: (a) estimating an amount of global ionospheric delay attributable to all of the tracked GPS satellites; and (b) for each tracked GPS satellite, estimating an amount of local ionospheric delay attributable thereto.
Also disclosed herein is a global positioning system comprising: (a) a GPS receiver configured to (1) receive a plurality of signals from a plurality of visible GPS satellites, and (2) produce a plurality of pseudorange measurements from the received signals, the pseudorange measurements being indicative of the GPS receiver""s position and having an amount of ionospheric delay error contained therein; (b) a processor configured to estimate the amount of ionospheric delay in the pseudorange measurements by (1) estimating an amount of global ionospheric delay attributable collectively to the plurality of visible GPS satellites, and (2) estimating a plurality of amounts of local ionospheric delays, each local ionospheric delay being attributable to a different visible GPS satellite.
Preferably the processor implements the ionospheric delay estimations using a modified Kalman filter. As noted above, the ionospheric delay amounts may be determined as a function of a single GPS frequency L1 or L2. The particular frequency used may change over time as the frequency available to the GPS receiver changes. Further still, the present invention will also operate when both GPS frequencies are available.
Preferably, the global ionospheric delay amount is initially set equal to a predetermined value and that initial value is subsequently modified as a function of an obliquity factor and a scalar, the scalar being dependent upon the single GPS frequency. The local ionospheric delay amounts are preferably initialized and updated in the same manner.
These and other advantages of the present invention will be in part apparent and in part pointed out in the following description, claims, and referenced figures.